This invention relates to an output correction method for exhaust gas ingredient-concentration sensors of a proportional-output type adapted for use in internal combustion engines, and more particularly to a method of this kind, which enables to properly correct variations in output characteristic between the individual sensors.
A method is known, in which exhaust gas concentration is sensed and the air-fuel ratio of a mixture supplied to an internal combustion engine is controlled to a desired ratio in a feedback manner responsive to the sensed concentration, so as to enhance the emission characteristics of the engine and reduce fuel consumption. As an 0.sub.2 sensor for sensing the concentration of oxygen contained in the exhaust gases, so called proportional-output type 0.sub.2 sensors are known, which generate an output proportional to the oxygen concentration.
0.sub.2 sensors of this type include a limiting current type comprising a cell element and an oxygen-pumping element, each composed of a plate-like member formed of a solid electrolytic material having oxygen ion-conductivity, and a couple of electrodes attached to opposite side surfaces of the plate-like member, and which senses the air-fuel ratio of the mixture e.g. by pumping oxygen into and out of a gas diffsion chamber defined between the cell and oxygen-pumping elements.
However, there can be variations in the output value between individual 0.sub.2 sensors due to variations in the diameter of a gas-introducing slit opening into the gas diffusion chamber, or variations in the composition or thickness between individual solid electrolytic materials, or variations in the heating temperature or processing time for forming the materials. These variations cause variations in the air-fuel ratio value detected by the individual 0.sub.2 sensors.
A method for correcting such variations in the air-fuel ratio is known e.g. from Japanese Provisional Utility Model Publication (Kokai) No. 60-120354, in which a correction resistance is connected as a circuit constant to a circuit connecting the oxygen-pumping element to a power source, wherein correction of variations in the output between individual sensors is carried out by changing the amount of current flowing in the circuit by means of the correction resistance.
Further, another method for correcting the variations has been proposed by the assignee of the present application in Japanese Provisional Pat. Publication (kokai) No. 62-198744, in which when the output level of the sensor is equal to a predetermined reference value, the amount of fuel supplied to the engine is actually changed by a predetermined amount, and a correction value is calculated on the basis of a change in the output of the sensor resulting from the change in the amount of fuel, and the correction value is used to correct the sensor, thereby enabling the correction merely by means of calculation.
However, the method according to Publication No. 60-120354 has the following disadvantage: In an 0.sub.2 sensor which can sense the air-fuel ratio over a wide range from the rich side to the lean side with respect to a stoichiometric, the output (pumping current Ip) characteristic of the sensor is generally such that the rate of a change in the pumping current Ip responsive to a change in the air-fuel ratio on the rich side is different from that responsive to a change of the air-fuel ratio on the lean side Therefore, the sensor output cannot be properly corrected on both the rich side and the lean side by means of a single correction resistance to thereby fail to detect the air-fuel ratio with accuracy. Consequently, this method requires the use of two correction resistances to correct the sensor output on both the rich side and the lean side.
On the other hand according to the method of Publication No. 62-198744, although the correction of the sensor output can be effected by means of calculation within a microcomputer, it is required to actually change the amount of fuel by multiplying a normally required amount of fuel by a predetermined value, which will make the engine control complicated.